1. Field of the Invention
The present invention relates to an optical scanning device for scanning a member with a light beam, and to an image forming apparatus such as a digital copier, a laser printer, and a laser facsimile machine using the optical scanning device.
2. Discussion of the Related Art
Optical scanning devices used for digital image forming apparatuses such as copiers, laser printers, and laser facsimile machines typically have a configuration such that a light beam emitted by a light source is deflected by an optical deflecting device, and the deflected light beam is then focused upon a surface of a member (such as a photoreceptor) by a scanning optical system such as fθ lenses to form a light spot thereon while scanning the member with the light beam in a main scanning direction.
There are full-color image forming apparatuses such that light beams emitted by plural light sources are deflected by an optical deflecting device; the deflected light beams are then focused upon surfaces of respective photoreceptors at the same time to form electrostatic latent images on the photoreceptors; the electrostatic latent images are developed with respective color developers having different colors such as yellow, magenta, cyan, and black color developers to form separate visible color images; the visible color images are sequentially transferred onto a recording sheet to form a combined multi-color image; and the combined multi-color image is fixed to the recording sheet by a fixing device to form a full color image. Among such color image forming apparatuses, tandem color image forming apparatuses using two or more sets comprised of an optical scanning device and a photoreceptor to produce multi-color images or full color images are well known. In particular, a tandem color image forming apparatus using a single optical deflecting device for forming electrostatic images on two or more photoreceptors is proposed as an example of such a tandem color image forming apparatus. Such a configuration can reduce the complexity and thus the cost as well as the overall size of the image forming apparatus overall.
Specific examples of such a tandem color image forming apparatus include the following:
(1) An optical scanning device in which multiple substantially parallel light beams separated in a sub-scanning direction are guided to an optical deflecting device to be deflected, and the deflected multiple light beams are guided to respective optical scanning elements to scan photoreceptors with the light beams; and
(2) An optical scanning device in which multiple light beams are guided to one side of an optical deflecting device, the deflected multiple light beams are converged in a sub-scanning direction by a first optical system, the multiple light beams are then converged in a main scanning direction by a second optical system, and the multiple light beams are guided to respective photoreceptors after passing mirrors and respective third optical systems deployed on the photoreceptors.
As another example of optical scanning devices using only one optical deflecting device to reduce costs thereof, an optical scanning device using an oblique incidence type optical system in which multiple light beams are guided to a surface of a deflecting device at different angles in the sub-scanning direction is known. In the oblique incidence type optical scanning device, multiple light beams deflected by the surface of the deflecting device are guided to surfaces of respective photoreceptors after being separated by a reflecting mirror or the like. In this regard, the angle of the light beams in the sub-scanning direction (i.e., the angle of the light beams incident on the surface of the optical deflecting device) is set to an angle at which the light beams can be separated from each other by such a reflecting mirror. By using such an oblique incidence type optical scanning device, multiple light beams can be separated without increasing the size of an optical deflecting device (i.e., without increasing the length of a polygon mirror in the sub-scanning direction).
Recently, in an attempt to produce high quality images, image forming apparatuses producing not only yellow, magenta, cyan and black color images, but also one or more other color images such as white toner images and transparent toner images using four photoreceptors (i.e., without increasing the number of photoreceptors) have been proposed. Although the number of photoreceptors is not increased in these image forming apparatuses, there is no space for an additional image forming module around a photoreceptor. Therefore, it is preferable to increase the number of photoreceptors so as to correspond to the number of color images produced.
To achieve low-cost, space-saving, and energy-efficient optical scanning devices for use in full color image forming apparatuses, oblique incidence type optical scanning devices are preferably used. However, oblique incidence type optical scanning devices have a drawback of causing scanning line bending such that the scanning light beams undergo a marked bending, resulting in formation of a misaligned multi-color image in which one or more color images are formed at positions different from the target positions thereof. The amount of bending of a scanning light beam varies depending on the oblique incidence angle of the light beam in the sub-scanning direction.
In a color image forming apparatus in which the number of photoreceptors is increased so as to be five or more to produce high quality images, not only the costs and size of the optical scanning device but also the costs and the size of the image forming apparatus as a whole are increased. In general, among various optical elements of an optical scanning device, the polygon mirror is more costly than the other optical elements. Even when using an optical scanning device having only one optical deflecting device, the optical deflecting device has to have a multi-tiered polygon mirror in which three or more polygon mirrors are overlaid in the sub-scanning direction to deflect multiple light beams toward respective photoreceptors, resulting in an increase in size of the optical deflecting device.
For these reasons, there is a need for an oblique incidence type optical deflecting device which has a low oblique incidence angle to prevent any increase in the amount of bending of a scanning light beam and wave aberration thereof, and which has low costs while saving space and energy.